In known apparatus for viewing the fundus and other particular areas within the vitreous body of the human eye, an inter-image of the viewed part of the eye is generated by means of an ophthalmoscopic lens; and this inter-image is then viewed by means of a surgical microscope. The image viewed in this manner is inverted vertically and laterally, and is pseudo-stereoscopic, i.e., depth perception is inverted from front to back. However, in order to be able to perform microsurgery, it is necessary to provide an image that is stereoscopically correct, namely "right side up". In addition to this required image erection, the two viewed optical paths must be interchanged (pupil interchange) in order to prevent the occurrence of the pseudo-stereo effect which would otherwise occur during stereoscopic viewing.
Image erection and pupil interchange may be achieved by positioning a prism system between the binocular tubes and the magnification changers of the surgical microscope. Such a prism system is described, for example, in U.S. Pat. No. 4,710,000 (Spitznas et al.). This additional module increases the working distance between the eye of the patient and the pupil of the viewer. However, for ergonomic and practical reasons, such increased working distance is undesirable in medical applications. Furthermore, such a prism system becomes quite complex in those commonly occurring situations when a second binocular tube must be provided to accommodate another viewer. With this type of prior art prism system, the second viewing tube requires its own appropriate second such prism system.
German Patent Application No. 35 39 009 and German Utility Model No. G 89 02 035.9 suggest a second possible solution in which the ophthalmoscopic lens and an appropriate prism system for reversing the generated inter-image are accommodated in an attachment to the surgical microscope. In this second prior art solution, the image-reversing prism system is located directly in front of the main lens of the surgical microscope, and the area of the eye of interest is focused with the actual surgical microscope which has a two-element main lens, one main lens element being shifted relative to the other for such focusing purposes. Therefore, each time this prior art attachment is installed or interchanged, the surgical microscope must be refocused, which is quite a problem during surgery. This second prior art has another disadvantage: The inversion system is positioned directly in front of the main lens and spatially separates the diverging stereo beam paths completely at a point where these paths are still partially overlapping. As a result of this, portions of these two optical paths are partially blocked out, causing image errors (vignetting) in the surgical microscope. Such a system has a further disadvantage in that the surgical microscope must have greater length to include a two-element main lens, thereby reducing the possible operating range available for the surgeon's use.
International PCT Application WO 91/15150 describes a third prior art solution where means for inverting the image is placed directly behind the side of the ophthalmoscopic lens which faces the surgical microscope. The inversion system comprises several prisms. Since the inversion system is positioned directly behind the ophthalmoscopic lens, the inter-image produced by the ophthalmoscopic lens lies within the prisms and has to be observed there through the surgical microscope. Therefore, the prisms used for inverting the image must be made with extreme accuracy and without optical errors. This third solution has a further disadvantage, namely, focusing is made by either adjusting the whole device in a vertical direction or by moving one of the inverting prisms relative to the others, both of which focusing systems require relatively complex mechanical arrangements. Furthermore, when vertically adjusted, the distance between the patient's eye and the ophthalmoscopic lens does not remain constant but rather depends on the focusing adjustment.
Therefore, the problem to be solved by the present invention is to fulfill the need for a surgical microscope with an ophthalmoscopic attachment that provides a vertical inversion of an inter-image produced by an ophthalmoscopic lens and that interchanges the optical viewing paths, while permitting the optional use of a conventional surgical microscope, either in a conventional manner or as an ophthalmoscope, without requiring changes to the surgical microscope. In so doing, such a single ophthalmoscopic attachment must assure that an inter-image without image errors can be viewed stereoscopically through the surgical microscope, even by a second viewer, by merely the addition of another conventional binocular tube to the conventional microscope.